FIG. 1 very schematically and partially shows in the form of blocks an example of a conventional switch-mode power supply of this type. In this example, the power source is an A.C. voltage Vac (for example, the mains voltage originating from the electric distribution network). Voltage Vac is rectified by a diode bridge 1 (for example, fullwave) having its rectified output terminals 2 and 3 connected by a capacitor Cp across which a smoothed D.C. voltage is present. This voltage is applied to the terminals of a primary winding 4p of a transformer 4 by being divided by a switch 5 in series with this winding. Switch 5 is controlled by a pulse train provided by a pulse-width modulation circuit 6 (PWM). It may also be a modulation of the pulse frequency.
On the secondary side (winding 4s) of transformer 4, a diode D in series with a capacitor Cs is connected across winding 4s. Capacitor Cs provides a voltage Vout for supplying a load 7 (Q) between output terminals 8 and 9 of the switch-mode power supply. Information about voltage Vout is further sampled (for example, between terminals 8 and 9) for a measurement circuit 10 (MES) constitutive of a regulation loop of the on periods of switch 5 according to a reference value of the supply voltage of load Q. Circuit 10 controls a photodiode PD of an optocoupler 11 having its phototransistor PT connected to a regulation circuit 12 (REG) intended to provide, to block 6 of generation of the pulse train, at least one first reference signal CT. A second signal OVL of detection of a possible overload, that is, of too high a current surge by load 7, is also provided to circuit 6 by circuit 12.
The example of FIG. 1 is that of a so-called “flyback” converter in which the power is transferred from the primary to the secondary of the circuit during periods where switch 5 is off. The present invention is however not limited to this type of converter and also applies to converters of “forward” type in which the power transfer is performed during on periods of the cut-off switch.
FIG. 2 shows a conventional example of a regulation circuit 12 having its output terminals 26 and 21 providing, to a pulse train generation circuit 6, respectively a regulation signal CT and an overload detection signal OVL. This circuit comprises, in series with phototransistor PT between a terminal 23 of application of a D.C. supply voltage Vcc and ground 24 on the secondary side, a capacitor C12. Circuit 12 being generally made in the form of an integrated circuit, the midpoint of this series association is directly connected to an input terminal 20 of regulation signal FB (that is, of connection of the emitter of phototransistor PT). An analog comparator 25 (differential amplifier) has its non-inverting input connected to terminal 20 and its inverting input which receives a fixed reference voltage VFB conditioning output voltage Vout of the converter. The output of comparator 25 controls a switch M (for example, a MOS transistor) which connects terminal 20 to terminal 26 of provision of a control current (signal CT) to circuit 6 (FIG. 1). A constant current source 27 further connects terminal 20 to ground 24.
The function of comparator 25 is to regulate the voltage of terminal 20 (and thus of the phototransistor emitter) to the value of voltage VFB.
When the load needs increase, voltage Vout tends to increase. Circuit 10 (FIG. 1) then controls emitting diode PD, which increases the base current of phototransistor PT. Assuming capacitor 12 to be charged (steady state), the current increase in phototransistor PT increases the voltage of the non-inverting input of comparator 25 since current source 27 cannot carry off more current (constant current). The output voltage of comparator 25 decreases and the current in transistor M increases. The current on terminal 26 forming the current control signal of pulse train generation circuit 6 increases and is interpreted by circuit 6 for, in the example of FIG. 1, decreasing the on periods of switch 5, to build up less power and thus decrease output voltage Vout.
If the load requires more power, voltage Vout tends to decrease. This decrease translates as a decrease in the current in the optocoupler, which tends to decrease the voltage of terminal 20. In fact, capacitor C12 discharges into current source 27, which causes an increase in the output voltage of comparator 25 and a decrease in the conduction of transistor M. The current on terminal 26 decreases and is interpreted by circuit 6 to increase the on periods of switch 5 to build up more power and increase output voltage Vout.
Circuit 12 comprises a second overload detection comparator 28. This comparator has its inverting input connected to terminal 20 and its non-inverting input which receives a voltage VOVL forming an overload threshold. The output of comparator 28 is connected to terminal 21 which provides an overload detection signal OVL to circuit 6.
In the presence of an overload, the greater current surge required at the output causes an abrupt reduction in the current in the optocoupler, which even eventually turns off. With no overload detection, terminal 26 would provide a signal CT requiring from circuit 6 to provide more current still. Now, in case of an overload, it is conversely appropriate to stop the power supply to the secondary.
The function of comparator 28 is to detect when the comparator can no longer maintain terminal 20 at voltage VFB by the regulation. Voltage VOVL is selected to be smaller than voltage VFB and comparator 28 switches when the discharge of capacitor C12 in current source 27 is such that terminal FB reaches threshold VOVL. In normal operation, the regulation does not let capacitor C12 discharge sufficiently, which prevents the triggering of comparator 28. Conversely to comparator 25, comparator 28 is generally a comparator with an all-or-nothing output.
In a regulation circuit such as illustrated in FIG. 2, capacitor C12 is used to set the intervention delay of comparator 28 after occurrence of an overload (turning-off of phototransistor PT). This delay is necessary to further enable starting of the circuit on powering-on thereof.
A problem is posed in certain devices (for example, of computer hard disk or printer head type) in which a transient or temporary current surge (for example, at the starting) must not be interpreted as an overload. With a conventional circuit such as illustrated in FIG. 2, a capacitor C12 sufficiently large to detect the detection has to be provided, to avoid starting comparator 28 in transient current surges. A disadvantage then is that the circuit assembly must be oversized to be able to stand transient overloads. Indeed, the power supply will constantly provide all the required power to the secondary, provided that the level remains smaller than the protection threshold corresponding to the regulation loss at the secondary. Now, this level generally corresponds to a level greater than the temporary overloads that the circuit must accept for a proper operation.